The present invention relates generally to telecommunication systems and, more specifically to radio communication systems for both voice and data transmission between two or more telecommunication units, at least one of which can be mobile.
In the past decade, various techniques and concepts of radio communication systems for mobile telephony and data transmission have been developed. In general, two types of mobile communication systems can be distinguished,
One is cellular communication, providing service to mobile subscribers which may move relatively fast through relatively wide areas, called cells. Analog cellular systems, such as designated AMPS, ETACS, NMT-450 and NMT-900 have been deployed throughout the world. Digital cellular systems are designated IS-54B, D AMPS in the Unites States, PDC in Japan and the pan-European GSM system.
Cordless radio communication is the other system, ranging from simple residential cordless telephones to business cordless systems capable of serving hundreds or even thousands of cordless communications units across (large) offices or production halls, etc. and local public communication. Analog cordless systems are designated CT0, CT1 en CT1+. Digital cordless systems are designated CT2, CT2-CAI, CT3, PHS and DECT.
GSM, D-AMPS, PDC and CT3, PHS and DECT use a digital access technique known as TDMA (Time Division Multiple Access), wherein transmissions take place in time slots and a plurality of time slots are gathered in a frame. Some emerging digital communication systems use CDMA (Code Division Multiple Access) as their access technique for establishing radio communication between communication units of a system. In general, these mobile or cordless communication systems are arranged for providing service at Multiple Carriers (MC) in a radio transmission band. That is, transmission at a particular carrier is provided using either TDMA or CDMA.
In cellular systems, like GSM, one or a plurality of carriers are individual to a cell. Cells that are spaced sufficiently apart can re-use the same carriers, without distorting effect. This so-called frequency re-use scheme allows an operator to serve a wide coverage area with many users and only a limited spectrum.
Cordless systems such as DECT and PHS are not based on frequency re-use. Instead, all the carriers are available for each cell. The system determines which carrier and communication channel of such carrier can be used for communication purposes, with the least amount of interference. Carriers and communication channels are dynamically allocated before and/or during transmission, which is called DCA (Dynamic Channel Allocation). Accordingly, no frequency planning or the involvement of an operator which controls the frequency spectrum are required.
For application in the home, like a wireless extension of the PSTN (Public Switched Telephone Network), or for wireless extensions of a LAN (Local Area Network), so-called unlicensed radio transmission bands are used. That is, subscribers do not require a license for operating radio equipment in such a radio transmission band. However, rules imposed on unlicensed bands by regulatory bodies like ETSI in Europe and the FCC in the U.S. prevent a single user of claiming the entire transmission band. Such rules usually involve collision avoidance schemes or spreading. In the case of spreading, the transmission of a subscriber is spreaded over the entire transmission band. Since different transmissions are uncoordinated, a radio air interface has to be applied which is robust against interference.
A global unlicensed transmission band has been defined at 2.4 GHz. Radio communication systems that are deployed in this unlicensed band have to adhere to strict rules defined by ETSI and the FCC. In the 2.4 GHz ISM (Industrial Scientific Medical) transmission band, spreading is mandatory. This means that a DCA schema used for DECT is not permitted. Instead FH (Frequency Hop) spreading or DS (Direct Sequence) spreading has to be applied, for example.
International Patent Application WO 93/17507 of applicant, which is herein included by reference, discloses several FH schemes for a TDMA radio communication system, wherein radio communication units transmit at radio channels in accordance with pseudo-random channel hopping schemes.
International Patent Application WO 93/22850 discloses a method of increasing interference diversity in a TDMA radio communication system, wherein frequency hopping is used and interference diversity is further increased in that communication channels also hop in a time slot raster. That is, instead of a fixed time slot allocation, time slots hop in position between sequential frames, i.e. TH (time slot hopping).
The ISM band is open to all equipment that fulfills the FCC and ETSI rules. Coordination between systems, to reduce interference, is not allowed, This means that the air interface of a communication system has to cope with unknown interference and jammers.
By applying any or both FH and TH schemes, the transmission of information in both frequency and time is randomised. That is, during a session, a communication channel occupies different frequency and time positions, such that the interference encountered at a communication channel is an average of all the channels at the transmission band. This is of particular advantage for voice transmission.
In practice, cellular and cordless radio communication systems both support voice and data transmission. In the context of the present description, the term voice communication is used for real time transmission of speech, whereas the term data communication is used for the transmission of other information, inclusive non-real time speech.
To enable error free delivery of data, data transmission typically uses a retransmission scheme to retransmit data packets that have been received in error. In an ARQ (Automatic Retransmission Query) scheme, the sender of the data is notified by the recipient about the performance of the previous transmission. If the sender has been informed that the reception was in error, the erroneous data is automatically retransmitted. In TDMA system like DECT, asymmetric data links can be established in which most of the time slots of a TDMA frame are allocated for simplex data transfer, i.e. in one direction only. However, at least one return communication channel has to remain in force to provide the ARQ information. To minimize delay and, accordingly, maximize throughput, the return channel directly follows the data channels. As a consequence, the recipient can acknowledge all data immediately preceding the return channel.
However, using a time slot hopping scheme, due to the random character of the hopping scheme, the return channel in a particular frame may end up in front of the data packets. Accordingly, immediate acknowledgment of received data packets in the same frame is not possible, simply because the return channel is not available after receipt of the data packets. Absence of the ARQ information results in an automatic retransmission of the previously transmitted data by the sender. This, despite the fact that the data might have arrived correctly. Those skilled in the art will appreciate that the throughput of the system is considerably reduced, and on average only 50% of the maximum throughput capacity is achieved.
It is an object of the present invention to provide a method of and equipment for radio communication, applying time hopping of time slots of a TDMA frame, suitable for coexistence of various types of communication in the same frame.
It is in particular an object of the present invention to provide an optimized allocation of time slots of a TDMA frame to support voice transmission in error prone radio communication systems, such as the unlicensed ISM band.
These and other objects, advantages and features of the present invention are provided by a method of radio communication in a TDMA radio communication system, wherein a communication channel comprises at least one time slot of a plurality of sequential time slots forming a TDMA frame and wherein a time slot of a frame hops in position between sequential frames. According to the present invention, the hopping of a time slot depends on the type of communication channel to which the time slot is allocated.
Following the method according to the invention, the hopping of time slots in a TDMA frame is communication channel dependent. That is, instead of pseudo-randomly hopping the position of the time slots in a frame, i.e. the communication channels to which the time slots are allocated, in the method according to the invention, the hopping of time slots is controlled by the type of communication channel, such as a voice communication channel or a data communication channel. In a preferred embodiment of the method according to the invention, hopping of time slots between sequential frames is restricted to time slots allocated to voice communication channels, whereas time slots allocated to data communication channels occupy fixed positions in a frame.
This embodiment of the method according to the invention provides optimum interference diversity for voice communication, whereas data transmission can be accomplished following an ARQ scheme. This, because the sequence of data communication channels is not affected by the method according to the invention, such that the return channel is always available after the data have been transferred.
It is noted that with this transmission scheme, the data communication channels are more vulnerable for interference than the voice communication channels. This, due to the absence of interference diversity for the time slots allocated to data communication channels. However, using an ARQ scheme, distorted data packets can be easily replaced by applying retransmissions.
In a further embodiment of the method according to the invention, in order to improve interference diversity for both voice and data communication channels, the radio communication system is arranged for radio communication in a predetermined radio frequency transmission band comprising a plurality of radio frequency transmission channels, and each such radio frequency transmission channel comprises a plurality of communication channels in a TDMA frame. Interference diversity is increased by having each TDMA frame hopping in position over the frequency transmission channels.
That is, by applying a frequency hopping scheme to the TDMA frames, for example such that each subsequent frame is transmitted in a subsequent radio frequency transmission channel, interference of data time slots is averaged over the radio frequency transmission band of the system.
In order to maintain time slots integrity, in a further embodiment of the invention, the hop positions of time slots allocated to voice communication channels are limited to time slots of a frame not allocated to data transmission channels.
In a preferred embodiment of the method according to the invention, a clear separation between time slots allocated to voice communication channels and time slots allocated to data communication channels is provided, in that time slots for data communication are allocated from one end of a frame and time slots for voice communication are allocated from another end of the same frame. Preferably, voice communication is assigned to the leading time slots of a frame or frame half and data communication is assigned to trailing time slots of a frame or frame half. That is to say, in a TDMA/TDD (Time Division Duplex) communication system data is received and transmitted in separate halves of a frame.
Clustering of time slots in consecutive trains, i.e. voice time slot clustering and data time slot clustering is advantagous with respect to interference, since it reduces the number of collisions due to partial overlap. In addition, a maximum space for the hopping of voice time slots is provided.
In accordance with a yet further embodiment of the invention, time slot hopping is achieved in that for each consecutive frame a different time offset is determined which is added to the time slots of the frame allocated to a voice communication channel, or in general the slots which may hop. The time offset comprises a multiple of a slot time.
The number of time slots positions available for hopping changes dynamically when time slots allocated for data communication are added or released in a frame. If a TDMA frame comprises N time slots, the time hop space M of a frame is provided by: M=Nxe2x88x92Ndxe2x88x921, wherein Nd represents the number of time slots of a TDMA frame assigned to data communication. In the case of a TDMA/TDD frame the time offset M is: M=N/2xe2x88x92Nxe2x80x2dxe2x88x921 wherein Nxe2x80x2d represents the largest number of time slots in a frame half of TDMA/TDD frame occupied by data communication channels. Note that due to asymmetric data communication channels, the number of time slots allocated to data communication may differ from one frame half to the another of a TDMA/TDD frame.
In a preferred embodiment of the method according to the invention, the hopping time slots wrap around time slot Mxe2x88x921 counted from the end of a frame or frame half of which the hopping time slots are assigned. Channels that are shifted outside the time hop space rotate back to this end of the frame. For TDMA/TDD the wrapping around procedure may be performed separately for each frame half.
For synchronisation purposes, the time offset of a frame is transferred to each communication unit of the radio communication system operative on a communication channel suitable for time slot hopping, such as a voice communication channel. In a radio communication system arranged for radio communication between a radio access unit or radio base station and a plurality of remote radio communication units, such as portable telephones and computer equipment, for example, the time offset is transferred from the radio access units to the remote radio communication units.
In the embodiments of the invention in which TDMA frames hop in position between radio frequency transmission channels of a radio frequency transmission band, fast synchronisation between radio communication units is achieved, in a still further embodiment of the invention, by having one unit continuously transmitting, such as the radio access unit, whereas the other unit scans a particular radio frequency transmission channel for the receipt of a transmission, and locks to the hopping of the frames at receipt of a transmission. If no communication channel is active, one or more so-called dummy bearers are transmitted, which are also time and frequency hopped.
Time hop synchronisation is accomplished by scanning all the time slots of a frame or frame half, until a message has arrived comprising information as to the time hop sequence and time offset.
For scanning purposes, in a yet further embodiment of the method according to the invention, the radio communication unit performing the scanning sweeps through the entire radio frequency transmission band, wherein a particular radio frequency transmission channel is scanned for a number of frames at least equal to the number of radio frequency transmission channels of the radio communication systems.
The invention further relates to a radio communication system comprising a plurality of radio access units and a plurality of remote radio communication units, each access unit providing radio communication service in a limited geographical area or cell. The radio access units and the remote radio telecommunication units are arranged for providing radio communication in accordance with the method disclosed above.
In a preferred embodiment of the radio communication system according to the invention, the radio access units transmit at a common radio frequency transmission band, and the radio access units each have an individual radio frequency transmission channel hopping sequence.
In a preferred embodiment of the invention, a radio communication system for providing radio communication in the Industrial Scientific and Medical (ISM) frequency band ranging from 2400 to 2483.5 MHz is provided, comprising 79 radio frequency transmission channels, each channel having a transmission bandwidth of 1 MHz, and wherein each TDMA frame has a length of 10 ms with a hopping rate of 100 hops/s over the radio frequency transmission channels, and wherein a TDMA/TDD transmission scheme is implemented in which each frame half comprises 12 consecutive time slots.
The invention further relates to a radio access unit and a radio communication unit of a radio communication system for cordless telecommunication, comprising transceiver means and control means for performing radio communication in accordance with the method disclosed above.
The above-mentioned and other features and advantages of the invention are illustrated in the following description with reference to the enclosed drawings.